NSFetchedResultsController

Introduction#

NSFetchedResultsController is a connection between core-data table (entity in core-data, table in sqlite) and UITableView. UITableView can be attached to any core-data entity using NSFetchedResultsController and UITableView will be updated as and when core-data updates that entity/table.

NSFetchedResultsController for UITableView

class ConversationsTableViewController: UITableViewController, NSFetchedResultsControllerDelegate {

private var fetchedResultsController: NSFetchedResultsController<Conversation>!

override func viewDidLoad() {
    super.viewDidLoad()
    initializeFetchedResultsController()
}
private func initializeFetchedResultsController() {
    let request = NSFetchRequest<Conversation>(entityName: "Conversation")
    let timeSort = NSSortDescriptor(key: "lastMessageTime", ascending: false)
    request.sortDescriptors = [timeSort]
    
    let MOC = AppManagedObjectContext()//It should be main thread MOC
    fetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: MOC, sectionNameKeyPath: nil, cacheName: nil)
    fetchedResultsController.delegate = self
    
    do {
        try fetchedResultsController.performFetch()
    } catch {
        print("Failed to initialize FetchedResultsController: \(error)")
    }
}

//table view methods
override func numberOfSections(in tableView: UITableView) -> Int {
    if let n = fetchedResultsController?.sections!.count {
        return n
    }
    return 0
}
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    let sectionInfo = fetchedResultsController.sections![section]
    let n =  sectionInfo.numberOfObjects
    return n
}
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "identifier", for: indexPath)
    //configure cell
    return cell
}

//NSFetchedResultsController Delegates
func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    tableView.beginUpdates()
}

func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange sectionInfo: NSFetchedResultsSectionInfo, atSectionIndex sectionIndex: Int, for type: NSFetchedResultsChangeType) {
    switch type {
    case .insert:
        tableView.insertSections(IndexSet(integer: sectionIndex), with: .fade)
    case .delete:
        tableView.deleteSections(IndexSet(integer: sectionIndex), with: .fade)
    case .move:
        break
    case .update:
        break
    }
}

func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
    switch type {
    case .insert:
        tableView.insertRows(at: [newIndexPath!], with: .fade)
    case .delete:
        tableView.deleteRows(at: [indexPath!], with: .fade)
    case .update:
        tableView.reloadRows(at: [indexPath!], with: .none)
    case .move:
        tableView.deleteRows(at: [indexPath!], with: .fade)
        tableView.insertRows(at: [newIndexPath!], with: .fade)
    }
}

func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    tableView.endUpdates()
}
}